BDateTime: Time_T functions return or take a time_t

* They used an unsigned int, which led to overflows when trying to set
them to a time before January 1st, 1970 (local time)
 * Some things use January 1st, 1970, GMT (or UTC) as a reference point.
In my timezone this leads to such a negative date. An example is cookie
expiration dates which are set to this date to expire them immediately.
Spotted by Opera testsuite.
 * This makes the method unuseable for dates after 2036 (signed 32-bit
time_t will overflow then. This gives us just 33 years to switch to a
64-bit time_t. In te meantime, please try using other methods to set the
date and time for BDateTime objects if you need to go this far.

headers/os/support/DateTime.h

@@ -189,8 +189,8 @@ public:
 			const BTime&		Time() const;
 			void				SetTime(const BTime &time);
 
-			int32				Time_t() const;
-			void				SetTime_t(uint32 seconds);
+			time_t				Time_t() const;
+			void				SetTime_t(time_t seconds);
 
 			bool				operator!=(const BDateTime& dateTime) const;
 			bool				operator==(const BDateTime& dateTime) const;